Two mirror re-imaging telescope

ABSTRACT

An infrared telescope utilizes two mirrors in an off-axis, eccentric-pupil, re-imaging configuration. To improve the image quality of traditional two mirror telescopes, the reflective surfaces of both the primary and secondary mirrors are ellipsoidal. The ellipsoidal surface of the primary mirror has a greater eccentricity than the ellipsoidal surface of the secondary mirror. The infrared light entering through the eccentric pupil strikes the ellipsoidal reflective surface of the primary mirror. The light is reflected from the primary mirror to the ellipsoidal reflective surface of the secondary mirror. An intermediate image of the object being viewed is formed between the primary and secondary mirrors. The light is reflected from the secondary mirror to an image plane. An aperture stop is located between the secondary mirror and the image plane. The image plane is typically located within a cold shield, to reduce the likelihood that stray light will reach the image plane.

FIELD OF THE INVENTION

The present invention relates generally to telescope systems, and moreparticularly, to two mirror re-imaging telescopes.

BACKGROUND OF THE INVENTION

Infrared imaging systems, such as infrared telescopes, are used in manyapplications where objects that emit infrared energy need to bedetected. Infrared energy that is imaged by an infrared imaging systemtypically has a wavelength from about 1 micrometer to about 26micrometers. Often these infrared telescopes are used in space-basedapplications. For example, such a telescope may be used in a globalmissile defense system to detect missiles for targeting purposes. Such atelescope may also be used in unmanned space vehicles, such as unmannedservice vehicles, for locating and identifying objects in space.

Space-based applications have very stringent volume and weightlimitations. As such, any systems used in space-based applications mustbe as physically compact and lightweight as possible while still meetingthe performance requirements of the application. Space-based infraredtelescopes must be physically compact and lightweight, while having theimage quality required by the application.

Three mirror and four mirror telescopes are known to provide good imagequality, but are less physically compact and weigh more than two mirrortelescopes. Two mirror telescopes are known to be more physicallycompact and lighter-weight compared to three and four mirror telescopes,but two mirror telescopes have poorer image quality. The image qualityof two mirror telescopes is often improved by the use of a correctorelement, similar to a corrective eyeglass lens. However, this correctorelement increases the weight of the two mirror telescope, and thisincreased weight is a significant drawback for space-based telescopes.

Re-imaging telescopes are one class of reflecting telescopes. Are-imaging telescope forms an intermediate image between the primarymirror and the secondary mirror. The final image is formed at the imageplane. In a telescope in which it is desirable to have an aperture stop(i.e., a physical structure that defines the volume of light thatreaches the image plane) near the image plane, a re-imagingconfiguration may be more physically compact because the light beam isless likely to walk across the mirrors in a re-imaging telescope.

One known type of re-imaging telescope is a Gregorian telescope. AGregorian telescope typically has a parabaloidal primary mirror and anellipsoidal secondary mirror. In a Gregorian telescope, the primarymirror is typically imaging or mapping a point at infinity (i.e., theobject being viewed, such as a star) to a finite point (i.e., theintermediate image). A parabaloidal mirror has one focus at infinity andone focus at a finite point in front of the mirror, therefore theprimary mirror of a Gregorian telescope would typically be parabaloidal.The secondary mirror of a Gregorian telescope is typically imaging ormapping a finite point (i.e., the intermediate image) to another finitepoint (i.e., the final image). An ellipsoidal mirror has two finitepoints of focus, therefore the secondary mirror of a Gregorian telescopewould typically be ellipsoidal. The user of an astronomical telescope istypically interested in having a better image quality at the center ofthe field of view. A Gregorian with a parabaloidal primary mirror givesa re-imaging telescope a better image quality at the center of the fieldof view and a poorer image quality at the edges of the field of view.Some users of space-based telescopes, particularly those used in aglobal missile defense system to detect missiles for targeting purposes,are typically interested in imaging a wider field of view than that ofastronomical telescopes, in order to increase the likelihood that thetelescope will locate missiles for targeting.

Some re-imaging telescopes use an off-axis, eccentric-pupilconfiguration. An eccentric-pupil telescope uses an entrance pupil(i.e., where light enters the telescope) that is physically offset fromthe optical axis. Light entering the telescope through the eccentricpupil would strike only the portion of a standard annular primary mirrorthat is similarly offset from the optical axis. Therefore, aneccentric-pupil telescope is constructed using only a portion of astandard annular primary mirror to conserve weight and reduce costs.Further, such an optical system is un-obscured which makes stray lighteasier to control. An off-axis telescope views the reflected image on anarea of the image plane that is slightly offset from the optical axis.The amount of this offset will typically vary depending on the specificrequirements of the telescope.

The off-axis, eccentric-pupil configuration used in some re-imagingtelescopes provides several benefits. Stray light is easier to controldue to this configuration. The eccentric pupil means there is no hole inthe middle of the telescope optics (i.e., the telescope is un-obscured).This in turn means that there is no hole in the middle of the light beamwithin the telescope. This generally makes stray light easier to controlbecause the telescope designer need not try to block that portion of thelight beam having the hole. Additionally, this configuration allows formore physical space for a focal plane and cooler. A telescope willtypically have a focal plane located at the image plane. The focal planeis typically an electronic device which receives and processes theimage. The focal plane of a telescope is typically surrounded by a coldshield which blocks light other than the light directly reflected by themirror closest to the focal plane from reaching the focal plane. Toincrease the ability of an infrared telescope to perceive infraredlight, the telescope may have a cooler to cool the focal plane relativeto the other structures of the telescope. Some infrared telescopes,however, may use an uncooled focal plane. Having an eccentric pupilallows the telescope designer to eliminate the unused part of theprimary mirror. Being off-axis tends to move the image plane down, awayfrom the remaining piece of the primary mirror. The resulting effect ofthese two conditions is that there is more physical space for the focalplane, the mounting structure, and the cooler adjacent to the remainingpiece of the primary (i.e., in the physical space that would otherwisebe occupied by the unused part of the primary mirror). If theeccentric-pupil, off-axis configuration were not used, the focal plane,mounting structure, and cooler would typically be located behind theprimary mirror and/or in the hole that would exist in the center of theprimary mirror. Finally, this configuration typically reducesaberrations, thus resulting in improved image quality.

Even in light of the existing telescope designs, there is a need for animproved telescope, such as an off-axis, eccentric-pupil two mirrorinfrared telescope, for space-based applications that provides adequateimage quality over a relatively wide field of view in a lightweight andphysically compact structure.

BRIEF SUMMARY OF THE INVENTION

An infrared re-imaging telescope is therefore provided that utilizes twomirrors in an off-axis, eccentric-pupil, re-imaging configuration. Byonly requiring two mirror surfaces, the weight of the telescope isadvantageously reduced and the telescope can be more physically compactin comparison to three- or four-mirror configurations. To overcome thepoorer image quality of traditional two mirror telescopes, thereflective surfaces of both the primary mirror and the secondary mirrorare ellipsoidal, thereby providing adequate image quality over arelatively wide field of view.

According to one embodiment of the present invention, the telescopeincludes an eccentric pupil through which infrared light enters thetelescope. The eccentric pupil is offset from the optical axis. Thetelescope also includes a primary mirror having an ellipsoidalreflective surface for reflecting infrared light. The telescope alsoincludes a secondary mirror having an ellipsoidal reflective surface forreceiving infrared light reflected by the primary mirror after formationof an intermediate image and for further reflecting the infrared light.The telescope also includes an image plane for receiving the infraredlight reflected by the secondary mirror.

In one embodiment of the present invention, the ellipsoidal surface ofthe primary mirror has a greater eccentricity than that of theellipsoidal surface of the secondary mirror. For example, in oneembodiment of the invention, the eccentricity of the primary mirror isbetween −0.80 and −0.95, and is preferably −0.93. In this embodiment,the eccentricity of the secondary mirror is between −0.20 and −0.35, andis preferably −0.32. In another embodiment, a ratio of the eccentricityof the ellipsoidal surface of the primary mirror to the eccentricity ofthe ellipsoidal surface of the secondary mirror is between 2:1 and 4:1.

In one embodiment of the present invention, the telescope may furtherinclude a structure defining an aperture stop that is disposed betweenthe secondary mirror and the image plane. This aperture stop may begenerally circular in shape, it may be generally rectangular in shape,or it may be generally trapezoidal in shape. In this embodiment, lightis reflected from the secondary mirror to the image plane through thisaperture stop, but light that strikes the structure forming the stopdoes not reach the image plane. As a result, only light reflecting froman area of the secondary mirror which corresponds to the shape of theaperture stop reaches the image plane. Similarly, only light reflectingfrom an area of the primary mirror which corresponds to the shape of theaperture stop reaches the image plane. One purpose of this aperture stopis to prevent light which is reflected from the outer perimeters of themirrors, which are more likely to have optical distortions, fromreaching the image plane. An overall purpose of the aperture stop is tochange the volume of light reaching the image plane. In the embodimentsin which the aperture stop is generally rectangular, the ratio of thelength of the sides of the aperture stop along the radial axis to thelength of the sides along the tangential axis is approximately 2:3,thereby blocking light from a larger radial portion of the primarymirror and reducing aberrations associated with the perimeter of theprimary mirror.

In one embodiment of the invention, the telescope has a ratio of alength of the telescope to a diameter of a primary aperture ofapproximately 3:1. In another embodiment of the invention, the telescopehas a field of view of between 0.7 and 1.5 degree, and preferably 1degree. In another embodiment of the invention, the telescope has afocal ratio of 3 or greater. In another embodiment of the invention, thecenter of the field of view is off-axis by between 0.08 and 0.12,degree, and preferably by 0.1 degree.

As such, the present invention provides a telescope that is light-weightand has a physically compact structure, at least in comparison to three-and four-mirror configurations. The telescope may therefore be utilizedfor space-based applications that demand light weight and physicalcompactness. Moreover, the inventive telescope may have an improvedimage quality over a relatively wide field of view compared to a typicaltwo-mirror telescope.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a side elevation schematic of an infrared telescope, inaccordance with one embodiment of the present invention;

FIG. 2 is a perspective view of a cold shield of an infrared telescope,in accordance with one embodiment of the present invention;

FIG. 3 is a perspective view of a cold shield and associated aperturestop of an infrared telescope, in accordance with one embodiment of thepresent invention; and

FIG. 4 is a perspective view of a cold shield and associated aperturestop of an infrared telescope, in accordance with another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

FIG. 1 is a side elevation schematic of an infrared telescope, inaccordance with one embodiment of the present invention. As shown inFIG. 1, infrared light in the form of beam path 12 enters telescope 10through eccentric pupil 14. As discussed above, pupil 14 is eccentricsince the pupil is offset from the optical axis 15, which is shown as adashed line in FIG. 1. As discussed above, an off-axis telescope viewsthe reflected image on an area of the image plane that is slightlyoffset from the optical axis. In the telescope of the present invention,this offset is approximately 0.1 degrees. Referring again to FIG. 1, thetelescope includes a primary mirror 16 that is mounted to the telescopehousing 17 via primary mirror mount 18. Primary mirror 16 is positionedsuch that light entering the telescope strikes the primary mirror and,in particular, an ellipsoidal surface 20 of the primary mirror.

The telescope 10 also includes a secondary mirror 22 and a field stop 30defining an opening positioned between the primary and secondary mirrorsfor preventing stray light (i.e., light not in beam path 12) fromimpinging upon the secondary mirror. The infrared light reflected fromellipsoidal surface 20 of primary mirror 16 passes through the openingin field stop 30 to secondary mirror 22. Intermediate image 28 is formedas beam path 12 is reflected from primary mirror 16 to secondary mirror22. Note that field stop 30 is trimming or cropping intermediate image28. Secondary mirror 22 is typically mounted to telescope housing 17 viasecondary mirror mount 24.

The secondary mirror 22 also includes an ellipsoidal surface 26 toreflect the infrared light to image plane 33. Focal plane 32 receivesand processes the image at image plane 33. Focal plane 32 is positionedwithin cold shield 34, such that cold shield 34 blocks stray light fromreaching focal plane 32. The cold shield 34 defines an opening 36(termed an aperture stop) that faces the secondary mirror through whichthe infrared light enters cold shield 34 and impinges upon focal plane32. Opening 36 controls the beam of light that is imaged on the focalplane 32. It should be appreciated that opening 36 could be formed by anindependent structure rather than by cold shield 34. It should also beappreciated that, in some embodiments, a cold shield may not be used. Astructure such as a baffle may be used instead of a cold shield.

Focal plane 32 and cold shield 34 are typically mounted to telescope 10via focal plane mounting assembly 38. In addition to providingstructural support for focal plane 32 and cold shield 34, focal planemounting assembly 38 would typically include a cooler to cool focalplane 32 relative to the other structures of telescope 10. Focal planemounting assembly 38 would also typically include the electricalconnections between focal plane 32 and the external environment, such asan avionics system of a space vehicle within which telescope 10 ismounted.

The telescope of the present invention would typically have a length toprimary aperture ratio of approximately 3:1. The telescope of thepresent invention would also typically have a field of view ofapproximately 1 degree, and a focal ratio of approximately 3 or greater,preferably 3.3. The telescope may be formed of beryllium, aluminum, orsilicon carbide, it may be formed of electroformed nickel, or it may beformed of any other suitable optical material.

In one exemplary embodiment of the telescope of the present invention,the primary mirror has a radius of 278.61 mm, an eccentricity of −0.93,an aperture of 170 by 120 mm, and is 70 mm off-axis. In this exemplaryembodiment, the secondary mirror is 202.28 mm from the vertex of theprimary mirror, has a radius of 94.84 mm, an eccentricity of −0.32, anaperture of 84 by 64 mm, and is 28 mm off-axis. The exemplary embodimenthas an aperture stop 125.12 mm from the vertex of the secondary mirror.The aperture stop forms a trapezoidal aperture with a short sidemeasuring 22 mm, a long side measuring 26 mm, and angled sides measuring15 mm. The aperture stop is 10.53 mm below the optical axis. Thisexemplary embodiment forms a final image at an image plane that is 67.02mm from the aperture stop. The telescope of the exemplary embodiment hasa field of view of 1.4 by 1.4 degrees, with an offset of −0.1 degrees,and an effective focal number of approximately 3.3.

It should be appreciated that the infrared re-imaging telescope of thepresent invention comprises only two mirrors with power and no more thantwo mirrors with power. However, the telescope may incorporate a flatfolding mirror in some embodiments, or it may incorporate a flatscanning mirror in front of the telescope.

FIG. 2 is a perspective view of a cold shield of an infrared telescope,in accordance with one embodiment of the present invention. FIG. 2illustrates cold shield 34 mounted to focal plane mounting assembly 38.In this embodiment, opening 36 encompasses the full diameter of that endof cold shield 34 that faces secondary mirror 22, such that opening 36forms an aperture stop. A cold shield with an integral aperture stop isillustrated in U.S. Pat. No. 6,024,458 to Lundgren.

Alternatively, the end of the cold shield that defines the opening coulddefine a smaller opening, such as by having a centrally extendingannular flange, so as to narrow the incoming light that reaches focalplane 32. This annular flange would thereby form a smaller aperture stopthan is formed by opening 36 in FIG. 2. The annular flange could form anaperture stop in one of many different shapes, such as round, square,rectangular, or trapezoidal, depending on the requirements of thetelescope. In this regard, FIGS. 3 and 4 are perspective views of coldshields of an infrared telescope, in accordance with two additionalembodiments of the present invention. In FIGS. 3 and 4, the end of coldshield 34 that faces secondary mirror 22 includes an additionalstructure to define a smaller aperture stop. The aperture stop restrictsa portion of beam path 12 from reaching focal plane 32. The portion ofbeam path 12 that is able to reach focal plane 32 corresponds to theshape of the stop. As discussed above, the purpose of this aperture stopis to prevent light which is reflected from the outer perimeter of themirrors from reaching the focal plane. The outer perimeter of themirrors is more prone to causing aberrations. As such, the aperture stopdefined by the end of the cold shield advantageously blocks light fromthis potentially distorted area of the mirrors. The orientation of theaperture stop defined by the end of the cold shield relative to a radialaxis and a tangential axis of the primary mirror is illustrated in FIGS.3 and 4.

In the embodiment illustrated in FIG. 3, aperture stop 40 has agenerally rectangular shape. The longer side 44 of the rectangularaperture stop is generally oriented along the illustrated tangentialaxis of the primary mirror, and as such may be called the tangentialside. The shorter side 46 of the rectangular aperture stop is generallyoriented along the illustrated radial axis of the primary mirror, and assuch may be called the radial side. In the illustrated embodiment, theratio of the length of the shorter side 46 to the length of the longerside 44 is about 2:3. As such, more light is blocked along the radialaxis than along the tangential axis, thereby blocking more light fromthe potentially distorted outer perimeter of the mirrors. It should beappreciated that the ratio of the length of the shorter side to thelength of the longer side may vary, typically from 1:1 to 1:2. As theshorter side decreases in size relative to the longer side, more lightis blocked along the radial axis but the image quality may decrease dueto diffraction.

In the embodiment illustrated in FIG. 4, aperture stop 42 has a shapethat is generally trapezoidal with rounded comers. The opposed longerside 48 and shorter side 52 are generally oriented along the illustratedtangential axis of the primary mirror, and as such may be called thetangential sides. The angled sides 50 are generally oriented along theillustrated radial axis of the primary mirror, and as such may be calledthe radial sides. In the illustrated embodiment, the ratio of the lengthof the angled sides 50 to the length of the longer side 48 is about 2:3,although this ratio may also vary. As such, more light is blocked alongthe radial axis than along the tangential axis, thereby blocking morelight from the potentially distorted outer perimeter of the mirrors. Asmentioned above, it should be appreciated that the aperture stop mayhave any one of several different shapes depending on the specifictechnical requirements of the telescope.

As described above, the reflective surfaces of both the primary andsecondary mirrors are ellipsoidal. This is a departure from a typicalGregorian astronomical telescope which has a parabaloidal primarymirror. The parabaloidal primary mirror of the typical Gregoriantelescope generally gives a better image quality at the center of thefield of view and a poorer image quality at the edges of the field ofview. In the telescope of the present invention, using an ellipsoidalprimary instead of a parabaloidal primary may worsen the image qualityin the center of the field of view and may improve the image quality atthe edges of the field of view, as compared to a typical Gregoriantelescope. Therefore, the telescope of the present inventionadvantageously has a wider field of view than a typical Gregoriantelescope with an image quality that is adequate for infrared imaging.This wider field of view increases the likelihood that the telescopewill locate missiles for targeting.

The ellipsoidal surfaces of the primary and secondary mirrors may bedefined by the eccentricity of the ellipsoidal surfaces. Eccentricity isa measure of how much a surface deviates from spherical, with a spherehaving an eccentricity of 0 and a parabaloid having an eccentricity of−1. In the telescope of the present invention, the primary mirror has aneccentricity closer to that of a parabaloid, while the secondary mirrorhas an eccentricity closer to that of a sphere. In one embodiment of theinvention, the eccentricity of the primary mirror is between −0.80 and−0.95, and is preferably −0.93. In this embodiment, the eccentricity ofthe secondary mirror is between −0.20 and −0.35, and is preferably−0.32. As such, the ratio of the eccentricity of the primary mirror tothat of the eccentricity of the secondary mirror is approximately 3:1.Having a primary mirror eccentricity of about −0.93 and a secondarymirror eccentricity of about −0.32 provides the desired wide field ofview with adequate image quality for infrared imaging.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. An off-axis, eccentric-pupil infrared telescope comprising: a primarymirror having an ellipsoidal reflective surface for reflecting infraredlight; a secondary mirror having an ellipsoidal reflective surface forreceiving infrared light reflected by the primary mirror after formationof an intermediate image and for further reflecting the infrared light;and an image plane for receiving the infrared light reflected by thesecondary mirror.
 2. The telescope of claim 1, wherein the ellipsoidalsurface of the primary mirror has an eccentricity of between −0.80 and−0.95.
 3. The telescope of claim 2, wherein the eccentricity of theellipsoidal surface of the primary mirror is −0.93.
 4. The telescope ofclaim 1, wherein the ellipsoidal surface of the secondary mirror has aneccentricity of between −0.20 and −0.35.
 5. The telescope of claim 4,wherein the eccentricity of the ellipsoidal surface of the secondarymirror is −0.32.
 6. The telescope of claim 1, wherein an eccentricity ofthe ellipsoidal surface of the primary mirror is greater than aneccentricity of the ellipsoidal surface of the secondary mirror.
 7. Thetelescope of claim 6, wherein a ratio of the eccentricity of theellipsoidal surface of the primary mirror to the eccentricity of theellipsoidal surface of the secondary mirror is between 2:1 and 4:1. 8.The telescope of claim 1, further comprising a structure defining anaperture stop disposed between the secondary mirror and the image plane.9. The telescope of claim 8, wherein the aperture stop has a shape thatis generally rectangular.
 10. The telescope of claim 9, wherein a ratioof a length of a shorter side of the aperture stop to a length of alonger side of the stop is 2:3.
 11. The telescope of claim 8, whereinthe aperture stop has a shape that is generally trapezoidal.
 12. Thetelescope of claim 11, wherein a ratio of a length of an angled side ofthe aperture stop to a length of a longest side of the pupil is 2:3. 13.The telescope of claim 1, wherein the telescope has a ratio of a lengthof the telescope to a diameter of a primary aperture of 3:1.
 14. Thetelescope of claim 1, wherein the telescope has a field of view ofbetween 0.7 and 1.5 degrees.
 15. The telescope of claim 1, wherein thetelescope has a focal ratio of 3 or greater.
 16. The telescope of claim1, wherein an angle between an axis of rotation of the primary andsecondary mirrors and the center of the field of view is 0.1 degree. 17.An off-axis, eccentric-pupil infrared telescope comprising: a primarymirror and a secondary mirror having respective reflective surfaces thatface one another, wherein the reflective surfaces of the primary andsecondary mirrors are ellipsoidal, and wherein the ellipsoidalreflective surfaces have different respective eccentricities; and animage plane optically downstream of the primary and secondary mirrors.18. The telescope of claim 17, wherein the eccentricity of theellipsoidal surface of the primary mirror is between −0.80 and −0.95.19. The telescope of claim 18, wherein the eccentricity of theellipsoidal surface of the primary mirror is −0.93.
 20. The telescope ofclaim 17, wherein the eccentricity of the ellipsoidal surface of thesecondary mirror is between −0.20 and −0.35.
 21. The telescope of claim20, wherein the eccentricity of the ellipsoidal surface of the secondarymirror is −0.32.
 22. The telescope of claim 17, wherein the eccentricityof the ellipsoidal surface of the primary mirror is greater than theeccentricity of the ellipsoidal surface of the secondary mirror.
 23. Thetelescope of claim 22, wherein a ratio of the eccentricity of theellipsoidal surface of the primary mirror to the eccentricity of theellipsoidal surface of the secondary mirror is between 2:1 and 4:1. 24.The telescope of claim 17, further comprising a structure defining anaperture stop disposed between the secondary mirror and the image plane.